Superconducting cables and similar superconducting equipment are configured of a large number of superconducting wires. When superconducting equipment is used, it has its internal superconducting filament(s) cooled to a critical temperature (Tc) or lower and to do so the superconducting equipment is immersed for example in liquid nitrogen, liquid helium or a similar liquid coolant and held at cryogenic temperature. In contrast, if the superconducting equipment is for example inspected, it is extracted from the liquid coolant and a gaseous coolant or the like having room temperature is introduced to surround the superconducting equipment to increase the superconducting equipment in temperature from cryogenic temperature to room temperature. Increasing the superconducting equipment having been immersed in the liquid coolant in temperature to room temperature, however, causes a superconducting wire, a constituent thereof, to balloon.
Ballooning is such a phenomenon that as temperature increases, a liquid coolant having entered a superconducting wire is gasified and not discharged externally, and as a result the superconducting wire's internal pressure increases and the superconducting wire thus expands. Ballooning is caused as follows: when superconducting equipment is immersed in a liquid coolant, the liquid coolant enters a superconducting wire through a pin hole or a similar defect existing in a surface of the wire, and as temperature increases, the liquid coolant is gasified and expands. If a balloon is caused at a portion the portion has a superconducting wire with a current path disrupted and thus invites an impaired critical current value and/or a similarly impaired superconducting characteristic(s). The phenomenon of ballooning is disclosed for example by L. Masur, et al., “Long Length Manufacturing of High Performance BSCCO-2223 Tape for the Detroit Edison Power Cable Project” (Non-Patent Document 1).
In order to prevent ballooning, before a superconducting wire is actually used an whether the superconducting wire has a defect or not is inspected. One such inspection (or test) is a test employing pressurized nitrogen. In this test, a superconducting wire is immersed for a predetermined period of time in a liquid coolant pressurized to approximately 1 MPa. The wire is thus cooled, and subsequently increased in temperature rapidly to room temperature and inspected for whether it has ballooning. In the test, whether a superconducting wire has ballooning or not is inspected to determine whether the wire is a defective wire.
Non-Patent Document 1: L. Masur, et al., “Long Length Manufacturing of High Performance BS CCO-2223 Tape for the Detroit Edison Power Cable Project”, IEEE Trans. Appl. Superconductivity., vol. 11, No. 1 pp. 3256-3260.